Measuring the Ultrasmall

HAMBURG, Germany -- Without measurement standards, our descriptions of the physical world would have no meaning. An essential standard defines atomic-scale dimensions, which are necessary to understand atomic and molecular structures.

The distance between atoms in a perfect crystal of silicon -- the silicon lattice constant -- has emerged as this standard, but it suffers from problems of reproducibility. Now an international team of researchers has suggested that the radiation from an excited isotope of iron may offer a more accurate and more easily reproducible standard.

The researchers mounted a channel-cut silicon crystal on a high-angular-resolution rotation stage and used it to select particular wavelengths of x-rays from the DESY synchrotron facility in Hamburg and the Advanced Photon Source at Argonne National Laboratory in Illinois. They targeted a 6-µm-thick foil containing iron-57 with the beam and monitored at which wavelengths it induced Mössbauer radiation, yielding a value of roughly 0.86 Å.

This is superior to the silicon standard for measuring ultrasmall distances because the lattice constant varies with ambient temperature and pressure and requires perfect synthetic crystals that are beyond the reach of most research labs. The gamma-ray technique does not have this problem, being easily and accurately reproducible at any synchrotron radiation facility.

Yuri V. Shvyd'ko of the University of Hamburg and a member of the team said that the next step is to demonstrate the utility of the technique. The group, which reported its findings in the July 17 issue of Physical Review Letters, plans to submit an account to the Journal of Synchrotron Radiation describing how it has measured the lattice parameters of Al2O3 and the gamma wavelengths of other Mössbauer isotopes using the new standard.